1. Field of the Invention
The invention relates to braking control apparatus and method for a vehicle and, more particularly, to an antiskid control apparatus and an antiskid control method.
2. Description of the Related Art
A known antiskid control apparatus for a vehicle, such as a motor vehicle or the like, is described in, for example, Japanese Patent Application Laid-Open Publication No. 2001-310726. The apparatus has a construction in which if the antiskid control is executed on one of right and left wheels when the vehicle is running on a so-called uneven friction-coefficient road surface where the road surfaces corresponding to the right and left wheels have different coefficients of friction, a generally-termed yaw control of curbing the increase in the brake pressure of the other one of the wheels and therefore minimizing the difference between the braking forces on the right and left wheels is performed.
According to the above-described antiskid control apparatus, if the antiskid control is executed on one of right and left wheels, the yaw control is executed on the other one of the wheels that is not subjected to the antiskid control, so that the difference between the braking forces on the right and left wheels is minimized. Therefore, the aforementioned antiskid control apparatus is able to reduce the yaw moment that acts on the vehicle due to the difference between the braking forces on the right and left wheels, and to improve the stability of the vehicle, compared with an antiskid control apparatus that does not employ the yaw control.
Generally, in such vehicles as motor vehicles and the like, the antiskid control is performed on the right and left front wheels independently of each other. If the road surface friction coefficients corresponding to the right and left wheels are different from each other, the brake pressures of the right and left wheels at the time of start of the antiskid control are also different from each other. In the antiskid control, the brake pressure is increased and decreased in accordance with the deceleration of the vehicle body. If the vehicle is running on an uneven friction-coefficient road surface, the deceleration of the vehicle body becomes substantially equal to an intermediate value between a vehicle body deceleration estimated from the speed of the wheel on the high-friction coefficient side and a vehicle body deceleration estimated from the speed of the wheel on the low-friction coefficient side. Therefore, if the antiskid control is performed on the high-friction coefficient side wheel as well, the amount of reduction of the brake pressure of the wheel becomes great relative to a value appropriate for the friction coefficient of the road surface. If the vehicle is running on an uneven friction-coefficient road surface, the braking force on the high-friction coefficient side wheels becomes dominant in the braking force on the entire vehicle.
Therefore, if the antiskid control is performed on the high-friction coefficient side wheel as well when the vehicle is running on an uneven friction-coefficient road surface, the braking force on the high-friction coefficient side wheel fluctuates relatively greatly. Therefore, in some cases, a driver needs to perform corrective steering corresponding to fluctuations in the braking forces on the high-friction coefficient side wheel. This problem is particularly remarkable in the case of a vehicle whose behavior is greatly affected by differences between the braking forces on the right and left wheels, for example, a vehicle having a short wheelbase and a high center of gravity.